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Studies of the Mg2NiMg2NiH4 system
210 Solid state reactions (piezochemistry NaAlHq + 2 NaH -
in a belt-type apparatus):
NasAlHe (25 kbar and 250 “C)
Purification. Unsolvated microcrystalline powder of pure LiAlH4 (or LiAlD4) is obtained by precipitation from diethylether solution using toluene. Thermal behaviour and thermodynamics. Thermodynamic properties were investigated by calorimetric measurements. We also performed thermogravimetric analysis and differential scanning calorimetry studies with special attention to phase transformations. LiAlHe exhibits the following thermal behaviour: melting LiAlHqs) 163 decomp. Li--4(l)
L@lHe(s)
-
~ 180
SC
“C
LiAlH 40) LiaAlHe(,j + 2Al + 3Hs 3LiH + Al + 3f2Ha
At the melting point, AH = 16.6 + 0.5 kJ mol-’ and AS = 38.0 J K-r mol-‘. For NasAlHe a polymorphic transition is observed (see below) at 525 K just before melting. Structural aspects. There is no evidence that the tetrahydridoaluminates belong to the same structural family. LiAlH4 possesses a monoclinic structure in which the coordination number (CN) of lithium and aluminium is respectively 5 and 4. Study of the P-T phase diagram shows the formation of a dense form in which the CN of aluminium is 6. NaAlH4 crystallizes with a scheelite-type structure (tetragonal CaW04) with CN for sodium of 8 and for aluminium of 4. The ionic environment of A13+ has been studied by Raman spectroscopy. Work on KAlH4 is now in progress; this compound probably possesses a barite-type structure (orthorhombic RaSO4) with CN for potassium of 12 and CN for aluminium of 4. Except for LiaAlHe, the hexahydridoaluminates crystallize with a cryolite (Na&lFe)or elpasolite (K~Na~F~) structure, generally pseudocubic. Under high temperature and/or high pressure conditions they may exhibit polymorphic transitions to more symmetrical structures. Neutron diffraction studies are in progress (for the deuterated compound) to try to elucidate the structure of Li3AlH6 which seems to be more complex.
Studies of the ~g~Ni-~g~N~~
system*
D. NOREUS Department of Structural Chemistry, S-l 06 91 Stockholm (Sweden)
Arrhenius
Laboratory,
University
of Stockholm,
A quantum chemical description of the transition metal-hydrogen complexes in MgsNiH4 and MgaFeHe has been made aiming at a fundamental understanding of the bonding in these systems, The low temperature phase of MgsNiH4 has been studied by electron microscopy. In this investigation the microtwinning, responsible for the earlier confusion in structural work was clearly seen. Finally, the o-Mg2NiHe.a + Hz -+ &MgsNiH4 reaction was studied using a volumetric method.
*Abstract of a paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Maubuisson, France, May 25 - 30,1986.
211 High accuracy quantum chemical calculations [ 1 ] (CASSCF and contracted CI) have been performed on the transition-metal-hydrogen complexes in the MgzFeH6 and MgzNiH4 crystals. The effects from the surrounding crystal are simulated by a Madelung field. If the conventional charge of +2 is assumed for the magnesium ions the transition-metalhydrogen complexes receive a charge of -4. Good agreement with experiment is then obtained for the bond distance and totally symmetric vibrational frequency for the iron complex, The calculated Fe-H distance in the octahedral FeHe4- complex is 1.56 A, which is identical to the value obtained from neutron diffraction data on the MgzFeHh crystal. For the nickel compound the unconventional case of Mg+ has also to be eonsidered, yielding a 2- transition-metal-hydrogen complex. The structure for the NiH4 complex is not as well determined as the iron complex. Experimental data has been interpreted in terms of a planar structure but a tetrahedral structure is not ruled out. Our calculations show that the conformation of the complex depends on the charge it is assumed to have. A 2- complex leads to a square planar structure and a 4- complex leads to a tetrahedral structure. The low temperature phase of MgzNiH4 has been studied by electron microscopy [ 21, From the electron diffraction pattern a monoclinic body-centered unit cell with the approximate parameters a = 6.5 8, b = 6.4 A, c = 13.2 A and fl= 93” could be deduced in agreement with earlier X-ray studies [ 31. Depending on the thermal history of the sample, microtwinning can be induced in the lattice with the twin planes parallel to (001). These stacking faults were further found to be pressure sensitive and can be reduced or eliminated if an isostatic pressure of the order of a few kbar is applied to the crystallites. The hydriding kinetics of MgaNi were studied at low temperatures where intrinsic processes dominate the reaction rates [4 J. A volumetric method was used to study the reaction oMgzNiHe_3 + Hz + /_I-MgaNiH4 Apg2 dependence was observed for the growth rate, and an Arrhenius activation energy of agout iO0 kJ mol-* could be calculated for the reaction. The kinetics were studied in the temperature range 360 to 440 K and with pressures below 25 bar.
1 P. Lindberg, D. Norbus, M. R. A. Blomberg and P. E. M. Siegbahn, J. Chem. Phys., to be published. 2 D. Nor&s and L. Kihlborg, J, Less-Common Met., 123 (1986) 233. 3 S. Ono, H. Hayakawa, A. Suzuki, K. Nomura, N. Nishimiya and T. Tabata, J. LessCommon Met., 88 (1982) 63. 4 S. Mattsoff and D. Nor&s, Int. J. Hydrogen Energy, to be published.
Preparation, structure and properties of Mg&oH,* P. ZOLLIKERa,
K. YVONa, P. FISCHERb and J. SCHEFERb
aLoboratoire de Cristallographie aux Rayons X, Universitk de Gem&e, 24, Quai E. Ansermet, CH-1211 Gensve 4 (Switzerland) bLabor fiir Neutronenstreuung, EidgenGssisch Xechnische Hochschule Ziirich, CH-5303 Wiirenlingen (Switzerland) We report the synthesis, structure and properties of a new hydride of composition MgsCoHs. The hydride and the deuteride were prepared from the elements magnesium and cobalt by a sintering technique at 380 - 420 “C and 40 - 60 bar hydrogen.
*Abstract of a paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Maubuisson, France, May 25 - 30,1986.
in a belt-type apparatus):
NasAlHe (25 kbar and 250 “C)
Purification. Unsolvated microcrystalline powder of pure LiAlH4 (or LiAlD4) is obtained by precipitation from diethylether solution using toluene. Thermal behaviour and thermodynamics. Thermodynamic properties were investigated by calorimetric measurements. We also performed thermogravimetric analysis and differential scanning calorimetry studies with special attention to phase transformations. LiAlHe exhibits the following thermal behaviour: melting LiAlHqs) 163 decomp. Li--4(l)
L@lHe(s)
-
~ 180
SC
“C
LiAlH 40) LiaAlHe(,j + 2Al + 3Hs 3LiH + Al + 3f2Ha
At the melting point, AH = 16.6 + 0.5 kJ mol-’ and AS = 38.0 J K-r mol-‘. For NasAlHe a polymorphic transition is observed (see below) at 525 K just before melting. Structural aspects. There is no evidence that the tetrahydridoaluminates belong to the same structural family. LiAlH4 possesses a monoclinic structure in which the coordination number (CN) of lithium and aluminium is respectively 5 and 4. Study of the P-T phase diagram shows the formation of a dense form in which the CN of aluminium is 6. NaAlH4 crystallizes with a scheelite-type structure (tetragonal CaW04) with CN for sodium of 8 and for aluminium of 4. The ionic environment of A13+ has been studied by Raman spectroscopy. Work on KAlH4 is now in progress; this compound probably possesses a barite-type structure (orthorhombic RaSO4) with CN for potassium of 12 and CN for aluminium of 4. Except for LiaAlHe, the hexahydridoaluminates crystallize with a cryolite (Na&lFe)or elpasolite (K~Na~F~) structure, generally pseudocubic. Under high temperature and/or high pressure conditions they may exhibit polymorphic transitions to more symmetrical structures. Neutron diffraction studies are in progress (for the deuterated compound) to try to elucidate the structure of Li3AlH6 which seems to be more complex.
Studies of the ~g~Ni-~g~N~~
system*
D. NOREUS Department of Structural Chemistry, S-l 06 91 Stockholm (Sweden)
Arrhenius
Laboratory,
University
of Stockholm,
A quantum chemical description of the transition metal-hydrogen complexes in MgsNiH4 and MgaFeHe has been made aiming at a fundamental understanding of the bonding in these systems, The low temperature phase of MgsNiH4 has been studied by electron microscopy. In this investigation the microtwinning, responsible for the earlier confusion in structural work was clearly seen. Finally, the o-Mg2NiHe.a + Hz -+ &MgsNiH4 reaction was studied using a volumetric method.
*Abstract of a paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Maubuisson, France, May 25 - 30,1986.
211 High accuracy quantum chemical calculations [ 1 ] (CASSCF and contracted CI) have been performed on the transition-metal-hydrogen complexes in the MgzFeH6 and MgzNiH4 crystals. The effects from the surrounding crystal are simulated by a Madelung field. If the conventional charge of +2 is assumed for the magnesium ions the transition-metalhydrogen complexes receive a charge of -4. Good agreement with experiment is then obtained for the bond distance and totally symmetric vibrational frequency for the iron complex, The calculated Fe-H distance in the octahedral FeHe4- complex is 1.56 A, which is identical to the value obtained from neutron diffraction data on the MgzFeHh crystal. For the nickel compound the unconventional case of Mg+ has also to be eonsidered, yielding a 2- transition-metal-hydrogen complex. The structure for the NiH4 complex is not as well determined as the iron complex. Experimental data has been interpreted in terms of a planar structure but a tetrahedral structure is not ruled out. Our calculations show that the conformation of the complex depends on the charge it is assumed to have. A 2- complex leads to a square planar structure and a 4- complex leads to a tetrahedral structure. The low temperature phase of MgzNiH4 has been studied by electron microscopy [ 21, From the electron diffraction pattern a monoclinic body-centered unit cell with the approximate parameters a = 6.5 8, b = 6.4 A, c = 13.2 A and fl= 93” could be deduced in agreement with earlier X-ray studies [ 31. Depending on the thermal history of the sample, microtwinning can be induced in the lattice with the twin planes parallel to (001). These stacking faults were further found to be pressure sensitive and can be reduced or eliminated if an isostatic pressure of the order of a few kbar is applied to the crystallites. The hydriding kinetics of MgaNi were studied at low temperatures where intrinsic processes dominate the reaction rates [4 J. A volumetric method was used to study the reaction oMgzNiHe_3 + Hz + /_I-MgaNiH4 Apg2 dependence was observed for the growth rate, and an Arrhenius activation energy of agout iO0 kJ mol-* could be calculated for the reaction. The kinetics were studied in the temperature range 360 to 440 K and with pressures below 25 bar.
1 P. Lindberg, D. Norbus, M. R. A. Blomberg and P. E. M. Siegbahn, J. Chem. Phys., to be published. 2 D. Nor&s and L. Kihlborg, J, Less-Common Met., 123 (1986) 233. 3 S. Ono, H. Hayakawa, A. Suzuki, K. Nomura, N. Nishimiya and T. Tabata, J. LessCommon Met., 88 (1982) 63. 4 S. Mattsoff and D. Nor&s, Int. J. Hydrogen Energy, to be published.
Preparation, structure and properties of Mg&oH,* P. ZOLLIKERa,
K. YVONa, P. FISCHERb and J. SCHEFERb
aLoboratoire de Cristallographie aux Rayons X, Universitk de Gem&e, 24, Quai E. Ansermet, CH-1211 Gensve 4 (Switzerland) bLabor fiir Neutronenstreuung, EidgenGssisch Xechnische Hochschule Ziirich, CH-5303 Wiirenlingen (Switzerland) We report the synthesis, structure and properties of a new hydride of composition MgsCoHs. The hydride and the deuteride were prepared from the elements magnesium and cobalt by a sintering technique at 380 - 420 “C and 40 - 60 bar hydrogen.
*Abstract of a paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Maubuisson, France, May 25 - 30,1986.